In this report we review the data on 75 male patients with extra Y chromosome diagnosed in Leuven in the period 1968-1993 among 98,725 patients (males and females) referred for constitutional chromosomal analysis. Special attention was given to their mental performance and psychosocial functioning. 1. Fifty male with 47,XYY karyotype were diagnosed. This is very close to the incidence of XYY in newborn studies and indicates that the frequency of MR/MCA is not increased in XYY male in general. 2. In the 60 patients with "pure" Y chromosome polysomy, the most frequent indication for karyotyping was the presence of MR and/or characterological problems in the index patients. Mental retardation was mostly borderline to mild, and severe mental retardation was rare. Characterological problems, difficulties in psychosocial integration and psychiatric problems were found in 86% of the mentally retarded versus 24% of the mentally normal men. 3. The 48,XXYY syndrome is characterized by markedly frequent and severe behavioural and psychiatric problems.